Electrophotographic marking is a well known method of copying or printing documents by exposing a substantially uniformly charged photoreceptor to an optical light image of an original document, discharging the photoreceptor to create an electrostatic latent image of the original document on the photoreceptor's surface, selectively adhering toner to the latent image, and transferring the resulting toner pattern from the photoreceptor, either directly to a marking substrate such as a sheet of paper, or indirectly after an intermediate transfer step. The transferred toner powder image is fused to the marking substrate using heat and/or pressure to make the image permanent. Finally, the surface of the photoreceptor is cleaned of residual developing material and recharged in preparation for the creation of the next image.
While many types of light exposure systems have been developed, a commonly used system is the raster output scanner (ROS) comprised of a laser beam, or beams, a means for modulating the laser beam (which, as in the case of a laser diode, may be the action of turning the source itself on and off) so that the laser beam contains image information, a rotating polygon mirror having one or more reflective surfaces, pre-polygon optics for collimating the laser beam, post-polygon optics to focus the laser beam into a well-defined spot on the photoreceptor surface and to compensate for the mechanical error known as polygon wobble, and one or more path folding mirrors to reduce the overall physical size of the scanner housing. The laser source, modulator, and pre-polygon optics produce a collimated laser beam which is directed to strike the reflective polygon facets. Some of these systems utilize a vertical cavity surface emitting laser (VCSEL) as the laser beam source.
As the polygon rotates, the reflected beam passes through the post-polygon optics and is redirected by any folding mirrors to produce a focused spot that sweeps along the surface of the charged photoreceptor in a straight scan line. Since the photoreceptor moves in a direction substantially perpendicular to the scan line, the swept spot covers the entire photoreceptor surface in a raster pattern. By suitably modulating the laser beam in accordance with the position of the exposing spot at any instant, a desired latent image can be produced on the photoreceptor.
However, imagers of this type frequently experience banding defects during the imaging process. Banding defects are defects in the latent image, in which multiple scan lines do not print evenly. This results in a visible banding effect, in which some scan lines in the final image appear lighter than other lines of theoretically equal tone.
Some of the most common sources of banding defects are due to the imager. Examples are ROS once-per-polygon revolution wobble, jitter, and beam to beam differences. Much of the work has focused on the various mechanical causes of banding and how interlacing schemes can decrease sensitivity to these banding sources. Some of the known mechanical sources of banding include magnification errors, array rotation, and beam non-uniformity.
However, all of the sources of blanding defects are not fully understood, and some banding defects seem to arise that defy predictive efforts and known mechanical solutions.